(read_char): After Fgarbage_collect, call redisplay.

(read_char): After Fgarbage_collect, call redisplay.

(read_key_sequence): When inserting `menu-bar' prefix,
modify the position field to prevent doing so twice.
Do all these forms of event expansion after replayed events also.
Set last_real_key_start before each key.
Use last_real_key_start in criterion for being the first event.

(syms_of_keyboard): Doc fix.
(Vhelp_char): Renamed from help_char.
(Vprefix_help_command): New Lisp variable.
(read_key_sequence): Use that, for help char after prefix key.

(kbd_buffer_get_event): Clear f before calling mouse_position_hook.